How can I define a default value for a field from the Shotgun web UI?

You can define a default value for a field via the configuration setting in the Status field. At the bottom you can see where you can set default values.

You can also set up a page with a Saved Filter for specific values.

As an example, if you have a filter like the one above set up on a page of Assets, whenever you create a new Asset, the Type field will be populated with “Prop” and the Description field will be pre-populated with “These are Props.”